It means that you are mediocre in your chosen major. If you can settle for such mediocre work, then you are just fine.
What does it mean if you keep on getting Cs or Ds in your major at university?
It means that you're not putting enough effort into your studies.
If you are already making a significant effort already to get high marks, I would consider that an indication that your natural insight and gifts might be better served with another choice of major.